Spreader deflection/mast stiffness

That was my original comment on not overdoing it on the the spreader length or you will get the mast moving to leeward at the spreaders which is not fast. But generally the relationship between spreader length and sideways stiffness of the rig is well established.

I don't really go with that theory. Try looking at it this way. If you have a mast with shrouds but no spreaders as the wind loads up the sail, the topmast will bend to leeward. When this happens the mast between the hounds and deck level will bend to windward, with the mast forming a smooth arc. If you now put a set of spreaders on they restrict the movement of the mast under the hounds to move to windward. This has the effect of reducing the movement of the tip of the mast to leeward. As you increase the length of the spreader it pushes more on the mast thus increasing the stiffening effect on the upper mast section. If this is overdone you can get an S- shape develop in the mast sideways, which normally means that you have overdone it on spreader length.  (Note Just because Adam Bowers, allegedly, once said that " increasing spreader length doesn't stiffen up the mast sideways" doesn't make it right!)

Think you need to recheck your sums... or maybe the theory behind them. Should be all about offsets on the spreaders from neutral. If the spreaders don't offset the shrouds at all they will have no compression on the mast at all. Usually the offset is only a few cm anyway, so in increase of 2cm must make a significant difference...

Doh!

Good job we have you to put us right Jim, otherwise I'd look like a right arse

You are of course right, and the effect on a boat such as a contender, is really dependant on sail shape and, rig tension, and spreader sweep.
If you look at the RS700 fleet, the general consensus early on was for the heavy guys to be running less swept but longer spreaders, however in reality guys at the top of the fleet are using setting that are rather varied. I for one have my sweep set to max. and the deflection set to one hole in from max. I did this simply to get my mainsail to set right upwind without too much fullness in the middle panels. At 84kg I'm at the heavier end of the 700 range (just), so need a bit of mast stiffness, but need a slightly flatter main than the fat boys thus the max. sweep.

In terms of setup I guess the contender is not too different.

Are you boys quite sure you aren't passing on some very class specific info as general advice? I have a hard time imagining a contender rig being powered up by shortening the spreaders. I think you're also missing the crucial role of rig tension on a trapeze boat with spreaders, especially a singlehanded one.